1. Field of the Invention
This invention relates generally to ultrasonic cleaning equipment, and relates more particularly to a regulated ultrasonic generator operable for supplying a driving signal to an ultrasonic transducer.
2. Description of the Relevant Art
The process of ultrasonic cleaning includes the steps of immersing a part to be cleaned in a suitable liquid medium, and agitating that medium with high-frequency sound energy for a short period of time. The high-frequency sound energy produces alternating rarefactions and compressions of the liquid. Small vapor cavities or bubbles form through cavitation during rarefactions and collapse during compressions. The formation and collapse of the vapor cavities create shock waves that impinge on the surface of the part and, through a scrubbing action, displace or loosen particulate matter.
The high-frequency sound energy is typically produced by some form of a displacement transducer, such as ferromagnetic or piezoelectric, that converts an electrical driving signal into mechanical motion. The electrical driving signal is generated and supplied to the ultrasonic transducer by an ultrasonic generator. One factor that affects the degree of scrubbing action of an ultrasonic cleaner is the frequency of the sound energy, which commonly ranges between 20 KHz and 120 KHz. The size and number of the cavitation cavities varies with the frequency of the sound energy, with higher frequencies producing more numerous cavities of smaller size than lower frequencies. The selection of an optimum frequency is difficult because it varies with each cleaning application.
Another factor that affects ultrasonic cleaning is the amplitude of the sound energy, which is proportional to the electrical energy supplied to the ultrasonic transducer. In order for cavitation to occur in a liquid medium, the amplitude of the sound energy must exceed a certain threshold value. The application of sound energy over and above this threshold value causes an increase in the overall quantity of the cavitation cavities, which may or may not be desirable for a particular cleaning application.
Still another factor that affects ultrasonic cleaning is the degree of entrapment of air in the liquid medium, which resists the collapse of the cavitation cavities and reduces the effectiveness of cleaning. The amount of entrapped air can be reduced by periodically switching off the ultrasonic transducer to permit adjacent air bubbles to coalesce, float to the surface, and escape, in a process known as degassing modulation.
Prior ultrasonic generators exhibit certain shortcomings that limit their effectiveness. One such shortcoming is that prior ultrasonic generators do not regulate the frequency and amplitude of the driving signal very closely, so that changes in the operational environment, such as the temperature or fluid level of the liquid medium, can produce an undesired shift in frequency or amplitude that, in turn, degrades cleaning performance. Another shortcoming is that many prior art ultrasonic generators do not offer protection against short circuit or open circuit operation. Under those conditions, such generators will blow fuses or even transistors.